1. Field of the Invention
This invention relates to a four stroke engine and more particularly to a camshaft drive arrangement most suitable to an outboard motor that has a protective cowling for the engine.
2. Description of Related Art
As is well known, a number of outboard motors, which are expected to produce a large horsepower, are provided with engines having V-shaped banks, each bank involving multiple cylinders. The cylinders are vertically spaced relative to each other and extend generally horizontally. Such a V-shaped configuration is quite suitable to outboard motors because power head of these motors can be small versus their engine powers. In addition to this, conventional outboard motors are mostly powered by two stroke engines. Since the two stroke engines are compact in nature, a power head accommodating this two stroke engine in the V-shaped configuration can be formed as small as possible.
Recently, however, some outboard motors incline to utilize four stroke engines. One reason for this tendency is that emissions of the four stroke engines are clean rather than that of two stroke engines. Generally, however, the four stroke engines have relatively complicated structures as compared with the two stroke engines. Particularly, if the engine is a DOHC (Double Over Head Camshaft drive) engine, it is provided with a relatively large size camshaft drive for activating intake valves and exhaust valves at its cylinder head assembly. This arrangement, thus, gives rise to making the cylinder head assembly be bulky. In addition, camshafts must rotate at a speed half as slow as that of the crankshaft in connection with activating timings of intake and exhaust valves. This means that the diameters of driven wheels such as pulleys or chain sprockets on the camshafts should be twice as large as the diameter of the crankshaft. Accordingly, the cylinder head assembly is likely to be more bulky.
In the meantime, usually a protective cowling encircles the engine in an outboard motor. Thus, the engine is desirable to be as small as possible for contributing to compactness of the outboard motor per se. However, the large driven wheels prevent this desire and the protective cowling tends to be large.
The aforedescribed situations will be described below with reference to a conventional, exemplary four stoke engine shown in FIG. 1.
FIG. 1 illustrates a plan view of the engine 10 and specifically a camshaft drive 12. This engine 10 has V-shaped cylinder banks 14a,b each having a few cylinders spaced vertically to each other. Each cylinder bank 14a,b has two overhead camshafts 16a,b (16c,d) disposed vertically for activating intake valves and exhaust valves. Thus, the engine 10 is a DOHC engine. At the bank 14a, which is located on the starboard (the left-hand side in the figure), the camshaft 16b positioned on the inside has a driven wheel 18 such as a pulley or a chain sprocket at its uppermost end. The camshaft 16b also has a driving wheel under the driven wheel 18, although it is not seen. Another camshaft 16a has a driven wheel 20 and an endless transmitter 22 such as a cog belt or a chain is wounded around the unseen driving wheel and the driven wheel 20. Meanwhile, the bank 14b located on the port side (the right-hand side in the figure) has a similar structure except that the camshaft 16d positioned on the outer side has a driven wheel 24.
Further, the engine 10 has a single crankshaft 26 extending vertically in the engine 10 and having a driving wheel 28 at its almost top end. An endless transmitter 30, like the transmitter 22, is wounded around the driving wheel 28 and the respective driven wheels 18, 24 of the camshafts 16b,d. With the rotation of the crankshaft 26, thus, the camshafts 16b,d are rotated and then the camshafts 16a,c are also rotated.
The driven wheels 18, 24 have the diameters twice as large as the diameter of the driving wheel 28. Because the camshafts 16a,b,c,d must rotate at a speed that is half as slow as that of the crankshaft in connection with activating timings of the intake and exhaust valves as described above.
On the other hand, although not shown, a protective cowling, which is generally tapered upwardly, encircles the engine 10. The large driven wheels 18, 24, particularly the wheel 24 positioned on the outer side, tend to make the protective cowling be large.
It is, therefore, a principal object of this invention to provide an improved DOHC engine contributing to compactness of an outboard motor accommodating the engine.
It is another object of this invention to provide a DOHC engine for an outboard motor, whereby a camshaft drive does not prevent a protective cowling encircling the engine from being formed compact.
Also, in order to minimize an outboard motor, an arrangement of an air induction system for a DOHC engine is quite important.
It is, therefore, a further object of this invention to provide a DOHC engine wherein an air induction system is arranged properly in view of the minimization of an outboard engine.
This invention is adapted to be embodied in a four stroke internal combustion engine for an outboard motor having a protective cowling encircling the engine.
In accordance with one aspect of this invention, at least two cylinders forming V-shaped banks are provided. Each cylinder includes a combustion chamber for burning intake charge. An intake valve is also included for admitting the intake charge into the combustion chamber. A first camshaft is further included for activating the intake valve. An exhaust valve is still further included for allowing the burnt charge being discharged from the combustion chamber. A second camshaft is yet further included for activating the exhaust valve. The first and second camshafts are disposed transversely relative to each other and generally vertically. A mechanism is also included for driving one of the first and second camshafts by another one of the first and second camshafts. A piston is reciprocally moved in the cylinder by burning of the intake charge in the combustion chamber. A single crankshaft rotated by the movement of the pistons is provided. The crankshaft is disposed generally vertically and apart from the respective camshafts. The crankshaft has a driving wheel. One of the first and second camshaft positioned on the inside relative to another one of the first and second camshafts in each of the banks has a driven wheel which diameter is larger than a diameter of the driving wheel. An endless transmitter is wound around the driving wheel and the driven wheels so that the driven wheels are driven by the driving wheel when the crankshaft is rotated by the movement of the pistons.
In accordance with another aspect of this invention, at least two cylinders forming V-shaped banks are provided. Each cylinder includes a combustion chamber for burning intake charge. An intake valve is also included for admitting the intake charge into the combustion chamber. A first camshaft is further included for activating the intake valve. An exhaust valve is still further included for allowing the burnt charge being discharged from the combustion chamber. A second camshaft is yet further included for activating the exhaust valve. The first and second camshafts are disposed transversely relative to each other and generally vertically. A mechanism is also included for driving the first and second camshafts. A piston is reciprocally moved in the cylinder by burning of the intake charge in the combustion chamber. At least one intermediate shaft is provided for activating the mechanism. A single crankshaft rotated by the movement of the pistons is provided. The crankshaft is disposed generally vertically and apart from the respective camshafts. The crankshaft has a driving wheel. The intermediate shaft has a driven wheel which diameter is larger than a diameter of the driving wheel. An endless transmitter is wound around the driving wheel and the driven wheel so that the driven wheel is driven by the driving wheel when the crankshaft is rotated by the movement of the pistons.
In accordance with a further aspect of this invention, at least two cylinders forming V-shaped banks are provided. Each cylinder includes a combustion chamber for burning intake charge. An intake valve is also included for admitting the intake charge into the combustion chamber. A first camshaft is further included for activating the intake valve. An exhaust valve is still further included for allowing the burnt charge being discharged from the combustion chamber. A second camshaft is yet further included for activating the exhaust valve. The first and second camshafts are disposed transversely relative to each other and generally vertically. The first camshafts are positioned on each outer side of the respective banks. The second camshafts are positioned on the inside of the respective banks. A piston is reciprocally moved in the cylinder by burning of the intake charge in the combustion chamber. A single crankshaft rotated by the movement of the pistons is provided. The crankshaft is disposed generally vertically and apart from the respective camshafts. A camshaft drive mechanism is provided for driving the first and second camshafts by the crankshaft when the crankshaft is rotated by the movement of the pistons. An air induction system is provided for supplying air that is one component of the intake charge through the intake valves. The air induction system includes at least one air chamber for taking the air from outside of the engine and being disposed apart from the intake valves. At least two delivery conduits are also included each for delivering the air to the combustion chambers. The delivery conduits are disposed at outer sides of the engine.
Further aspects, features and advantages of this invention will be become apparent from the detailed description of the preferred embodiments which follow.